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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>1.1WebGL 基础概念</title>
    <style>
        @import url("https://webglfundamentals.org/webgl/resources/webgl-tutorials.css");

        body {
            margin: 0;
        }

        canvas {
            width: 100vw;
            height: 100vh;
            display: block;
        }
    </style>
</head>

<body>
    <canvas id="c"></canvas>
    <script id="vertex-shader-2d" type="notjs">

  // an attribute will receive data from a buffer
  attribute vec4 a_position;

  // all shaders have a main function
  void main() {

    // gl_Position is a special variable a vertex shader
    // is responsible for setting
    gl_Position = a_position;
  }

</script>
    <script id="fragment-shader-2d" type="notjs">

  // fragment shaders don't have a default precision so we need
  // to pick one. mediump is a good default
  precision mediump float;

  void main() {
    // gl_FragColor is a special variable a fragment shader
    // is responsible for setting
    gl_FragColor = vec4(1, 0, 0.5, 1); // return redish-purple
  }

</script><!--
for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
See https://webglfundamentals.org/webgl/lessons/webgl-boilerplate.html
and https://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
for webgl-utils, m3, m4, and webgl-lessons-ui.
-->
    <script src="../lib/webgl-utils.js"></script>
    <script>
        /* eslint no-console:0 consistent-return:0 */
        "use strict";
        // 创建着色器方法，输入参数：渲染上下文，着色器类型，数据源
        function createShader(gl, type, source) {
            var shader = gl.createShader(type); // 创建着色器对象
            gl.shaderSource(shader, source); // 提供数据源
            gl.compileShader(shader); // 编译 -> 生成着色器
            var success = gl.getShaderParameter(shader, gl.COMPILE_STATUS);
            if (success) {
                return shader;
            }

            console.log(gl.getShaderInfoLog(shader));
            gl.deleteShader(shader);
        }
        // 将这两个着色器 link（链接）到一个 program（着色程序）
        function createProgram(gl, vertexShader, fragmentShader) {
            var program = gl.createProgram();
            gl.attachShader(program, vertexShader);
            gl.attachShader(program, fragmentShader);
            gl.linkProgram(program);
            var success = gl.getProgramParameter(program, gl.LINK_STATUS);
            if (success) {
                return program;
            }

            console.log(gl.getProgramInfoLog(program));
            gl.deleteProgram(program);
        }

        function main() {
            // Get A WebGL context
            var canvas = document.querySelector("#c");
            var gl = canvas.getContext("webgl");
            if (!gl) {
                return;
            }

            // Get the strings for our GLSL shaders
            var vertexShaderSource = document.querySelector("#vertex-shader-2d").text;
            var fragmentShaderSource = document.querySelector("#fragment-shader-2d").text;

            // create GLSL shaders, upload the GLSL source, compile the shaders
            var vertexShader = createShader(gl, gl.VERTEX_SHADER, vertexShaderSource);
            var fragmentShader = createShader(gl, gl.FRAGMENT_SHADER, fragmentShaderSource);

            // Link the two shaders into a program 然后我们将这两个着色器 link（链接）到一个 program（着色程序）
            var program = createProgram(gl, vertexShader, fragmentShader);

            // look up where the vertex data needs to go.
            var positionAttributeLocation = gl.getAttribLocation(program, "a_position");

            // Create a buffer and put three 2d clip space points in it
            var positionBuffer = gl.createBuffer();

            // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
            gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

            var positions = [
                0, 0,
                0, 0.5,
                0.7, 0,
            ];
            gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

            // code above this line is initialization code.
            // code below this line is rendering code.

            webglUtils.resizeCanvasToDisplaySize(gl.canvas);

            // Tell WebGL how to convert from clip space to pixels
            gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

            // Clear the canvas
            gl.clearColor(0, 0, 0, 0);
            gl.clear(gl.COLOR_BUFFER_BIT);

            // Tell it to use our program (pair of shaders)
            gl.useProgram(program);

            // Turn on the attribute
            gl.enableVertexAttribArray(positionAttributeLocation);

            // Bind the position buffer.
            gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

            // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
            var size = 2;          // 2 components per iteration
            var type = gl.FLOAT;   // the data is 32bit floats
            var normalize = false; // don't normalize the data
            var stride = 0;        // 0 = move forward size * sizeof(type) each iteration to get the next position
            var offset = 0;        // start at the beginning of the buffer
            gl.vertexAttribPointer(
                positionAttributeLocation, size, type, normalize, stride, offset);

            // draw
            var primitiveType = gl.TRIANGLES;
            var offset = 0;
            var count = 3;
            gl.drawArrays(primitiveType, offset, count);
        }
        main();
    </script>
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